Precision motors for driving, for example, a video head drum in video recorders are normally designed as externally commutated synchronous motors. These motors are driven with square-wave voltages by a digital controller, whose circuitry can easily be implemented.
The motor is designed, for example, with a rotor, which is designed as a disc magnet with two pole pairs. It is mounted such that it can rotate at a short distance above the stator coil, which is split into three coils, which are offset through 120.degree.. A magnetic return path which is mounted on the same axis guides the lines of force through the stator coils at right angles.
It is disadvantageous in the case of this motor drive that high currents occur in particular at the switching edges of the square-wave signals, and can lead to severe noise being produced, as well as to electrical interference. In addition, the torque profile is not sufficiently uniform. This is problematic, in particular at high rotation speeds of 9000 revolutions per minute.